Devices of this type are disclosed, for example, in Patent Literature 1. Each of the devices of Patent Literature 1 is provided with a coil antenna that is used for both of communication and power transmission. A power transmission system used between devices of Patent Literature 1 is an electromagnetic induction system that transmits power from a coil antenna of a device of a power transmission side to a coil antenna of a device of a power receiving side using magnetic flux as a medium. Especially, a power receiving device of the Patent Literature 1 is provided with an input connection circuit between a coil antenna and a communication circuit. The input connection circuit varies voltage applied to the communication circuit according to voltage occurred on the coil antenna. Thereby, it is prevented to apply overvoltage to the communication circuit at the time of power transmission.
As another wireless power transmission system that uses a coil, a resonance system, which utilizes resonance of circuits including coils, is known for example. Furthermore, a system in which an electrode of a power transmission side and an electrode of a power receiving side are arranged in proximity to each other to execute power transmission therebetween using capacitive coupling is also known (See Patent Literature 2).